Contrasting Effects of Alanine and Methionine on Nitrogen Ammonification and Nitrification, and Nitrous Oxide Emissions in Subtropical Forest Soil

The purpose of this study was to evaluate the impacts of two amino acids that contrast in their composition: alanine as a neutral amino acid (represent regular amino acid) and methionine (represent sulfur-containing amino acids) on nitrogen (N) dynamics in acidic subtropical forest soil. (15) N-labeled alanine and methionine were added into forest soil to carry out subsequent incubation experiments for 72 h under 60% and 90% of water-holding capacities (WHC) to investigate their potential impacts. The results showed that under 60% of WHC, alanine and methionine added treatment significantly increased the net ammonification rate, except at the 5th h for alanine addition due to alanine immobilized then to prime the soil N ammonification. Alanine addition induced NO3--N immobilization before 24 h and then nitrification occurred dominantly between 24 and 72 h. However, methionine addition resulted in a lower NO3--N compared to control indicating that nitrification was likely inhibited. Only methionine added treatment enhanced significantly the net N2O emission rate as compared to that in the control at the 72nd h due to nitrifier denitrification possibly. Under 90% of WHC, the net ammonification rate was significantly higher in the alanine added treatment than in the control, but lower in the methionine added treatment at the 5th h. Amino acids addition significantly decreased the net nitrification rate at the 5th h, but greatly increased the net N2O emissions rate at the 24th h and 72nd h compared to control. Overall, more than 96% of NH4+-N, NO3--N, and N2O emissions were recorded from soil native N rather than from the added amino acids. Contrasting effects of amino acids should be considered when studying the mechanisms of soil organic N in contributing to the N transformation in subtropical forest soil.